In the handling of certain types of combustible materials in particulate or fibrous form, such as cotton or wood particles, it is often convenient to transfer it from one point to another by blowing it through ducts. In such operations it has been found that occasionally a piece of material travelling in the duct will become ignited, possibly due to sparks caused by entrained metallic pieces striking pieces of rotating process machinery.
Although the material may not burst into flame, a smoldering ember will be produced which can eventually result in a serious fire in a mass of stored material at the discharge point of the duct.
Photo-optical detectors have been utilized in an attempt to detect such embers; however, types of which applicant is aware have been found not entirely reliable in detecting such embers, in that they are unstable and often have inadequate sensitivity.
The material being handled often travels at a speed of 50 feet per second, hence an ember travelling near a side wall of the duct may be in the view of the detector for only a few milli-seconds.
Detectors for this purpose of which applicant is aware utilize photoresistive devices as the detector element, and utilize the decrease in resistance of said element to a predetermined value to cause an output alarm signal.
The detection circuitry must be such as to render the device very sensitive to enable it to detect the small radiation output from an ember; however, resistance changes may also occur in such devices as a result of temperature changes and variations in ambient light. Permanent changes also occur as a result of aging. Changes from these effects may reach or exceed the resistance change caused by a passing ember.
Hence the device cannot be operated at its maximum sensitivity because of the possibility of false alarms due to resistance changes caused by the above-mentioned conditions. Detectors of this type, being responsive to ambient light, also require that the carrier enclosures be absolutely light tight.